Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstocks. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.
The petrochemical industry has known that oxygenates, especially alcohols, are convertible into light olefin(s). There are numerous technologies available for producing oxygenates including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Syngas production processes are well known, and include conventional steam reforming, autothermal reforming or a combination thereof.
Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reaction of hydrogen, carbon monoxide and/or carbon dioxide in a reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular reactor. The preferred methanol conversion process is generally referred to as a methanol-to-olefin(s) process (MTO), where an oxygenate, typically mostly methanol, is converted into primarily ethylene and/or propylene in the presence of a molecular sieve.
There are many different types of molecular sieves well known to convert a feedstock, especially an oxygenate containing feedstock, into one or more olefin(s). Molecular sieves, such as zeolites or zeolite-type molecular sieves, carbons and oxides, are porous solids having pores of different sizes that selectively adsorb molecules that can enter the pores, and exclude other molecules that are too large. Examples of molecular sieves useful in converting an oxygenate into olefin(s) are U.S. Pat. No. 5,367,100 [describes the use of ZSM-5 to convert methanol into olefin(s)]; U.S. Pat. No. 4,062,905 (discusses the conversion of methanol and other oxygenates to ethylene and propylene using crystalline aluminosilicate zeolites, for example, Zeolite T, ZK5, erionite and chabazite); U.S. Pat. No. 4,079,095 (describes the use of ZSM-34 to convert methanol to hydrocarbon products such as ethylene and propylene); U.S. Pat. No. 4,310,440 (describes producing light olefin(s) from an alcohol using a crystalline aluminophosphates, often represented by ALPO4); and U.S. Pat. No. 4,440,871 [describes silicoaluminophosphate molecular sieves (SAPO), one of the most useful molecular sieves for converting methanol into olefin(s)].
Typically, molecular sieves are formed into molecular sieve catalyst compositions to improve their durability in commercial conversion processes. The collisions within a commercial process between catalyst composition particles themselves, the reactor walls, and other reactor systems cause the particles to breakdown into smaller particles called fines. The physical breakdown of the molecular sieve catalyst composition particles is known as attrition. Problems develop in the recovery systems because fines often exit the reactor in the product containing effluent stream. Catalyst compositions having a higher resistance to attrition generate fewer fines; this results in improved process operability, and less catalyst composition being required for a conversion process, and therefore, lower overall operating costs.
It is known that the way in which the molecular sieve catalyst compositions are made or formulated affects catalyst composition attrition. Molecular sieve catalyst compositions are formed by combining a molecular sieve and a matrix material usually in the presence of a binder. For example, PCT Patent Publication WO 03/000413 A1, published Jan. 3, 2003 discloses a low attrition molecular sieve catalyst composition using a synthesized molecular sieve that has not been fully dried, or partially dried, in combination in a slurry with a binder and/or a matrix material. Also, PCT Patent Publication WO 03/000412 A1 published Jan. 3, 2003, discusses a low attrition molecular sieve catalyst composition produced by controlling the pH of the slurry away from the isoelectric point of the molecular sieve. U.S. Pat. No. 6,787,501 shows making a low attrition molecular sieve catalyst composition by making a slurry of a synthesized molecular sieve, a binder, and optionally a matrix material, wherein 90 percent by volume of the slurry contains particles having a diameter less than 20 μm. U.S. Patent Application Publication No. 2003/0181322, published Sep. 25, 2003, which is fully incorporated herein by reference, illustrates making an attrition resistant molecular sieve catalyst composition by controlling the ratio of a binder to a molecular sieve. U.S. Pat. No. 6,503,863 is directed to a method of heat treating a molecular sieve catalyst composition to remove a portion of the template used in the synthesis of the molecular sieve. U.S. Pat. No. 6,541,415 describes improving the attrition resistance of a molecular sieve catalyst composition that contains molecular sieve-containing recycled attrition particles and virgin molecular sieve. U.S. Pat. No. 6,660,682 describes the use of a polymeric base to reduce the amount of templating agent required to produce a particular molecular sieve.
It is also known that in typical commercial processes flocculants are used to facilitate the recovery of synthesized molecular sieves. Thus, without flocculants, the natural settling of the as-synthesized molecular sieve from the synthesis mixture may be too slow to be economically viable. In addition, the direct filtration of the synthesis mixture is often slow and inefficient because of the small crystal size of the as-synthesized molecular sieve and the presence of unconverted reactants and by-products that may also be small in size. Flocculants are therefore typically used to ensure faster and more complete settling of the as-synthesized molecular sieve from the synthesis mixture. However, the presence of excess flocculant can raise the viscosity of the synthesis mixture thereby making subsequent filtration more difficult. In addition, the presence of excess flocculant in the recovered molecular sieve can affect catalyst formulation, and in some cases can result in the formulation of catalyst compositions having lower attrition resistance, lower selectivity in various conversion processes, and high slurry viscosity. At present the determination of the amount of flocculant to be used in the recovery of molecular sieves involves a large amount of trial and error and hence there is a need for a method that facilitates the determination of optimal flocculant levels.
For an example of the use of flocculants in the recovery of molecular sieves, reference is directed to U.S. Patent Application Publication No. 2005/0256354, published Nov. 17, 2005, which claims a process for producing one or more olefin(s), comprising the steps of (a) introducing a feedstock to a reactor system in the presence of a molecular sieve catalyst composition comprising a synthesized molecular sieve having been recovered in the presence of a flocculant; (b) withdrawing from the reactor system an effluent stream; and (c) passing the effluent gas through a recovery system recovering at least the one or more olefin(s). See also, U.S. patent application Ser. No. 11/109,397, filed Apr. 19, 2005, which claims a catalyst composition comprising a non-over flocculated molecular sieve and an over flocculated molecular sieve. See also, U.S. patent application Ser. No. 11/109,584, filed Apr. 19, 2005, which claims a catalyst composition comprising an over flocculated molecular sieve and a phosphorous compound. See also, U.S. patent application Ser. No. 11/109,586, filed Apr. 19, 2005, which claims a catalyst composition prepared by the process comprising (a) combining: (i) an over flocculated molecular sieve; (ii) a binder; and (iii) optionally, a matrix material to form a catalyst slurry; and (b) milling the catalyst slurry.